Strobe acquisition and tracking

ABSTRACT

A memory controller includes an interface to receive a data strobe signal and corresponding read data. The data strobe signal and the read data correspond to a read command issued by the memory controller, and the read data is received in accordance with the data strobe signal and an enable signal. A circuit in the memory controller is to dynamically adjust a timing offset between the enable signal and the data strobe signal, and control logic is to issue a supplemental read command in accordance with a determination that a time interval since a last read command issued by the memory controller exceeds a predetermined value.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/959,633, filed Aug. 5, 2013, which was a continuation of U.S. patentapplication Ser. No. 12/520,068, filed Jun. 18, 2009, now U.S. Pat. No.8,504,788, which was a U.S. National Stage Application filed under 35U.S.C. §371 of PCT Patent Application Serial No. PCT/US2007/088244 filedon Dec. 19, 2007, which claimed the benefit of and priority to U.S.Provisional Application Ser. No. 60/876,408 filed on Dec. 20, 2006, allwhich are hereby incorporated by reference in their entireties.

FIELD

The subject matter disclosed herein relates generally to circuits foruse in integrated circuits, and in particular, to circuits andassociated methods for synchronizing strobe signals and data signals.

BACKGROUND

Many communication systems include devices that communicatesynchronously with one another. For example, some memory componentsutilize a strobe signal to provide source synchronous clocking eventsfor read and write information on the data signals. Unfortunately, asthe signaling or data rate is increased in such systems delays betweenthese signals may occur.

In particular, the total communication path length between memorycomponents is often relatively long. In the case of a read operation,the complete path includes generating the read command and addressinformation in the clock domain of a memory controller (which isreferred to as a PClk domain), transmitting this information acrossinterconnect links between the memory controller and a memory device,receiving the information at the memory device, performing the readoperation, transmitting read data signals and a corresponding strobesignal across the interconnect links to the memory controller, andreceiving the read data using the strobe signal at the memorycontroller. This communication path includes various delays that maychange as conditions vary, for example, there may be variations intemperature or a supply voltage.

The resulting delays between the data signals and the strobe signals maymake it difficult for the memory controller to determine when a timingevent on the strobe indicates the presence of read data, as opposed tonoise. Existing approaches to this challenge include manual tuning ofstrobe-enable-window circuits to achieve reasonable timing for eachsystem and silicon process. However, such adjustments are fixed, aretime consuming and difficult to perform, and have limited efficacy. Assuch, these approaches sacrifice timing margin. In addition, theseapproaches may not be capable of supporting systems that have unmatchedstrobe drifts greater than ±1 bit time.

There is a need, therefore, for improved timing drift cancellationcircuits and techniques that reduce and/or eliminating timing driftbetween data signals and strobe signals without the aforementionedproblems.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, reference should be made to the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1A is a block diagram illustrating an embodiment of a system.

FIGS. 1B and 1C are exemplary timing diagrams for the system shown inFIG. 1A.

FIG. 2A is a block diagram illustrating an embodiment of a circuit.

FIG. 2B is a block diagram illustrating an embodiment of a lock detectcircuit.

FIG. 2C is a block diagram illustrating an embodiment of a burst errordetect circuit.

FIG. 2D is a block diagram illustrating an embodiment of a delay adjustcircuit.

FIGS. 3A and 3B are exemplary timing diagrams for the circuit shown inFIG. 2A.

FIG. 4 schematically illustrates an eye pattern.

FIG. 5A is a block diagram illustrating an embodiment of a circuit.

FIG. 5B is a block diagram illustrating an embodiment of a circuit.

FIGS. 6A, 6B, 6C, 6D and 6E are exemplary timing diagrams for thecircuits shown in FIGS. 5A and 5B.

FIG. 7 is a flow diagram illustrating an embodiment of a method oftiming drift cancellation.

FIG. 8 is a block diagram illustrating an embodiment of a system.

Like reference numerals refer to corresponding parts throughout thedrawings.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of a memory controller are described. In some embodiments,the memory controller includes a first interface to receive a datastrobe signal and corresponding read data. The data strobe signal andthe read data correspond to a read command issued by the memorycontroller, and the read data is received in accordance with the datastrobe signal and an enable signal. A circuit in the memory controlleris to dynamically adjust a timing offset between the enable signal andthe data strobe signal, and control logic is to issue a supplementalread command if a time interval since a last read command issued by thememory controller exceeds a first predetermined value.

The data strobe signal and the corresponding read data may be receivedfrom a memory device, and the read command and the supplemental readcommand may be transmitted to the memory device. In some embodiments,the memory controller is to transmit and receive data on a first edge ofa clock and a second edge of a clock.

The dynamic adjustment may be to approximately center the data strobesignal within a window corresponding to the enable signal. In someembodiments, the circuit is to dynamically adjust the timing offset,between the enable signal and the data strobe signal, to less than asecond predetermined value in a first mode of operation. The secondpredetermined value may be one-half of a clock cycle.

The control logic may be to select an operating mode of the circuit inaccordance with a read data error associated with the timing offset. Insome embodiments, at least two consecutive read commands are to beissued during the first mode of operation.

In some embodiments, the timing offset may initially be larger than 1clock cycle. In some embodiments, the timing offset may initially belarger than 2 clock cycles.

In some embodiments, the dynamic adjustment of the timing offset in thefirst mode of operation is in accordance with a received patterncorresponding to the data strobe signal and a predefined pattern, wherethe predefined pattern includes a data strobe signal preamble. In someembodiments, the dynamic adjustment of the timing offset in the firstmode of operation is in accordance with a count value that correspondsto a number of edges in the data strobe signal during the window.

In some embodiments, the dynamic adjustment of the timing offset in thefirst mode of operation is in accordance with at least one pass-failboundary at which an error between a predefined read data pattern and areceived read data pattern occurs, where at least the one pass-failboundary corresponds to an extremum of the timing offset. In someembodiments, the dynamic adjustment of the timing offset in the firstmode of operation is in accordance with a first pass-fail boundary and asecond pass-fail boundary, a respective pass-fail boundary correspondsto a respective timing offset, and where the respective pass-failboundary is in accordance with the predefined read data pattern and thereceived read data pattern.

In some embodiments, an on-die termination in the first interface thatdefines a voltage level is to be biased during the first mode ofoperation. For example, a voltage of a data strobe signal in the firstinterface may be biased.

In some embodiments, the circuit is to dynamically adjust the timingoffset such that it remains less than the second predetermined value ina second mode of operation. The dynamic adjustment of the timing offsetin the second mode of operation may be in accordance with a receivedpattern corresponding to the data strobe signal and a predefinedpattern, where the predefined pattern includes a data strobe signalpreamble. Alternatively, the dynamic adjustment of the timing offset inthe second mode of operation may be in accordance with a time of a lastfalling edge in the data strobe signal.

In another embodiment, a method includes receiving the data strobesignal and the corresponding read data, where the data strobe signal andthe read data correspond to the read command issued by the memorycontroller, and the read data is received in accordance with the datastrobe signal and the enable signal. The method further includesdynamically adjusting the timing offset between the enable signal andthe data strobe signal, and issuing the supplemental read command if thetime interval since the last read command issued by the memorycontroller exceeds the first predetermined value.

In another embodiment, a memory controller includes an interface toreceive a data strobe signal and read data, where the data strobe signaland the read data correspond to a read command issued by the memorycontroller, and the read data is received in accordance with the datastrobe signal and an enable signal. The memory controller includescircuitry to dynamically adjust a timing offset between the enablesignal and the data strobe signal. In a first mode of operation, thedynamic adjustment of the timing offset may utilize a predefined datastrobe signal preamble. In some embodiments, the dynamic adjustment ofthe timing offset utilizes a count value, which corresponds to thenumber of edges in the data strobe signal during the window defined bythe enable signal. In some embodiments, the dynamic adjustment of thetiming offset is based on the time of a last falling edge of the datastrobe signal. And in some embodiments, the dynamic adjustment of thetiming offset utilizes at least one pass-fail boundary for limiting thetiming offset between the enable signal and the data strobe signal.

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings. In the following detaileddescription, numerous specific details are set forth in order to providea thorough understanding of the subject matter presented herein.However, it will be apparent to one of ordinary skill in the art thatthe subject matter may be practiced without these specific details. Inother instances, well-known methods, procedures, components, andcircuits have not been described in detail so as not to unnecessarilyobscure aspects of the embodiments.

Embodiments of circuits in a memory controller and a related method aredescribed. The memory controller receives read data using a data strobesignal and an enable signal. The memory controller is configured and/orconfigurable to synchronize the timing of the data strobe signal and theread data by dynamically adjusting a timing offset between the enablesignal and the data strobe signal. This dynamic adjustment mayapproximately center the data strobe signal within a windowcorresponding to the enable signal. The dynamic adjustment may be basedon matching at least a portion of the data strobe signal with apredefined pattern, counting edges in the data strobe signal during thewindow, and/or one or more pass-fail boundaries at which an errorbetween a predefined read data pattern and a received read data patternoccurs.

The memory controller may have two modes of operation. During a firstmode of operation, synchronization between the data strobe signal andthe read data is acquired. In some embodiments, on-die termination of aninterface that defines a voltage level in the memory controller isbiased during the first mode of operation. During a second mode ofoperation, the synchronization may be maintained by tracking the datastrobe signal and the read data. If the timing offset exceeds ±1 clockcycle or bit time, the memory controller may revert to the first mode ofoperation.

In some embodiments, the memory controller issues a supplemental readcommand to a memory device if a time interval since a last read commandexceeds a predetermined value, such as 10 ms, or 100 ms, or a valuebetween 10 ms and 100 ms. In this way, a minimum read command density orrate is achieved. This allows the dynamic adjustment to be repeatedsufficiently often to reliably compensate for drift in the timingoffset, such as drift that may be caused by or associated withtemperature, process and/or voltage variations.

The memory controller may be included in a system with at least onememory device. In some embodiments, a memory controller or memory devicetransmits and receives data on both the rising and falling edges of aclock, such as in dual data rate (DDR) systems. The memory device mayinclude a memory core that utilizes solid-state memory, semiconductormemory, organic memory and/or another memory material, includingvolatile and/or non-volatile memory. The memory device may includedynamic random access memory (DRAM), static random access memory (SRAM)and/or electrically erasable programmable read-only memory (EEPROM). Inembodiments with one or more memory devices, the memory devices may beembedded in one or more memory modules and/or two or more memory devicesmay be configured as a memory bank. The memory controller and the one ormore memory devices may be on a common or same circuit board. Thecircuit may be included in one or more components in other systems, suchas those that include logic chips, including a serializer/deserializer(SERDES), PCI Express and/or other high-speed interfaces or input/outputlinks. In some embodiments, at least a portion of the aforementionedcircuits may be included in one or more components in a memory system,such as one or more memory controllers and/or one or more memorydevices.

We now discuss embodiments that address the difficulties associated withexisting approaches for timing drift cancellation. FIG. 1A is a blockdiagram illustrating an embodiment of a system 100. A memory controller110 is coupled to a memory device 124 by links 132. In some embodiments,the links 132 correspond to wired and/or wireless communication. Thelinks 132 may be used for bi-directional and/or uni-directionalcommunications between the memory controller 110 and the memory device124. Bi-directional communication may be simultaneous. In someembodiments, one or more of the links 132 and corresponding transmitcircuits Tx 112 and/or receive circuits Rx 118 may be dynamicallyconfigured, for example, by control logic 136, for bi-directional and/orunidirectional communication.

Data may be communicated on one or more of the links 132 using one ormore sub-channels, such as a baseband sub-channel corresponding to afirst frequency band and/or a passband sub-channel corresponding to asecond frequency band. In some embodiments, such as those where at leastone of the links 132 is ac-coupled, the baseband sub-channel may notcontain DC (i.e., does not include 0 Hz). In some embodiments, the firstfrequency band and the second frequency band may be orthogonal. In otherembodiments there may be substantial overlap of one or more neighboringpairs of frequency bands. A respective sub-channel may also correspondto a group of frequency bands. In some embodiments, the data is encoded,for example, using one or more types of multiple pulse-amplitudemodulation (PAM), such as 2-PAM and/or 4-PAM.

Transmit circuits (Tx) 112-1 and 112-2 in the memory controller 110transmit a clock signal (PClk) 114 and a read command (RD) 116 acrossthe links 132 to the memory device 124. In the memory device 124, theclock signal 114 and the read command 116 are received using receivecircuits (Rx) 118-3 and 118-4, and are coupled to command interface 126.The read command may be performed by accessing appropriate addresses inmemory 130, such as a memory core. Corresponding read data is coupled toread interface 128.

Transmit circuits 112-3 and 112-4 of the memory device 124 transmit readdata signals and data strobe signals corresponding to the read dataacross the links 132 to the memory controller 110. Receive circuits118-1 and 118-2 of the memory controller 110 receive the data strobe andread data signals. These signals are coupled to a first-in first-out(FIFO) memory 120, where read data 122 is determined using these signalsand the clock signal 114.

As discussed previously, delays may occur at various components andlocations along this communication path, and these delays may varyand/or change based on processing conditions, as well as due to changesin temperature and/or one or more supply voltages. This is illustratedin FIG. 1B, which provides a timing diagram 140 for data (DQ) 150 andcorresponding data strobes (DQS) 152. Note that each transition in thedata strobes 152 is associated with corresponding data 150. Also notethat in some embodiments, four data bits or symbols are transmitted witheach data strobe signal 152. In other embodiments, the number of databits or symbols transmitted with each data strobe signal 152 may be morethan four (e.g., eight) or less than four. As illustrated in the timingdiagram 140, round trip delays T_(RD) vary between minimum 154-1 andmaximum values 154-2, which may be several cycles of the clock signal114 in the clock domain of the memory controller 110 (FIG. 1A).

In addition to global delays that apply to both the data 150 and thedata strobes 152, there may also be relative delays. In some systems,these relative delays may be more than 1 cycle of the clock signal 114.This is illustrated in the timing diagram 160 shown in FIG. 1C. Timingdrift 176 may occur between data 150-3 and data strobe 152-3. This drift176 may complicate detection of the data 150-3 and/or result in dataerrors.

In particular, detection circuitry in the memory controller 110 (FIG.1A) may generate additional signals, such as a data strobe internal(dqs_int) 170 and enable signal (dqs_en) 172. Data strobe internal 170may include multiple zeros and ones even when read data is not beingreceived. These spurious zeros and ones may occur because, at thesetimes, the data strobe is parked or set at an intermediate voltage nearthe switching point of the receive circuits 118-1 (FIG. 1A) and 118-2(FIG. 1A). As a consequence, factors such as coupled noise may give riseto transitions on data strobe internal 170. A timing windowcorresponding to the enable signal 172 may be used to clean up oreliminate these spurious zeros and ones to generate a clean version ofthe data strobe (dqs_clean) 174, which may be used to detect the data150-3. Thus, in order to receive the data 150-3, the detection circuitrysynchronizes the data strobe 152-3 and the enable signal 172, i.e.,reduces and/or eliminates a timing offset between these signals. Thissynchronization may be performed one or more times, such as after apredetermined time interval or dynamically (i.e., continuously).

Referring back to FIG. 1A, the detection circuitry in the memorycontroller 110 may include comparison circuit 134 and control logic 136.The comparison circuit 134 may determine the timing offset between theread data, the data strobe signal, and/or internal signals such as theenable signal 172 (FIG. 1C). The control logic 136 may select a mode ofoperation of the memory controller 110. In a first mode of operation thememory controller may acquire and track the data strobe signal, while inthe second mode of operation the memory controller may maintainsynchronization between the read data and the data strobe signals. Thecontrol logic 136 may switch between the first mode of operation and thesecond mode of operation based, either directly or indirectly, on driftor changes in a timing offset, such as the offset between the datastrobe 152-3 (FIG. 1C) and the enable signal 172 (FIG. 1C). For example,the control logic 136 may select the second mode of operation if thetiming offset in the first mode of operation is less than apredetermined value, such as one half of a cycle of the clock signal114. Alternatively, the control logic 136 may select the first mode ofoperation based on a read data error associated with the timing offset.Or the control logic 136 may select the first mode of operation if thetiming offset in the second mode of operation exceeds a predeterminedvalue, such as one half of a cycle of the clock signal 114.

As is also discussed further below, the detection circuitry in thememory controller 110 may be able to synchronize one or more of thesesignals with one another even when an initial timing offset is greaterthan ±1 or more cycles of the clock signal 114. In particular,adjustment, such as dynamic adjustment, of the timing offset between thedata strobe 152-3 (FIG. 1C) and the enable signal 172 (FIG. 1C) mayapproximately center the data strobe signal 152-3 (FIG. 1C) in a windowcorresponding to the enable signal 172 (FIG. 1C). Embodiments of suchcircuits for performing these adjustments are described further belowwith reference to FIGS. 3-6.

In embodiments where the adjustment is dynamic, a minimum read commanddensity or rate may be needed. In particular, since the adjustment onlyoccurs when a read data is received by the memory controller 110, ifread commands are issued infrequently excessive timing drift may occur.For example, the timing drift may have a time constant that isassociated with thermal and/or voltage changes, and if read commands areissued on a time scale longer than this time constant, excessive timingdrift may result. In some embodiments, therefore, the control logic 136may issue one or more supplemental read commands to the memory device124 if a time interval since a last read command exceeds a predeterminedvalue. In an exemplary embodiment, the predetermined value is betweenapproximately 10 ms and approximately 100 ms. Furthermore, in someembodiments at least two consecutive read commands are issued by thememory controller 110 during the first mode of operation. This isdiscussed below with reference to FIGS. 6A and 6B.

In some embodiments, the system 100 may include fewer or additionalcomponents, logical positions of one or more components may be changed,and two or more of the components may be combined and/or shared. Forexample, in some embodiments there may be one or more additional memorycontrollers and/or one or more additional memory devices. In someembodiments, the memory device 124 includes control logic.

FIG. 2A is a block diagram illustrating an embodiment of a circuit 200that may be included in the memory controller 110 (FIG. 1A). The circuit200 includes detection circuitry to synchronize the strobe enable signal172 and data strobe 210 during the first and/or the second mode ofoperation. The circuit 200 locks the negative or falling edge of theenable signal 172 with the last falling edge of the data strobe 210. Thephase of the data strobe 210 is sampled at the falling edge of theenable signal 172 (see latch 216-2). The samples indicate if the datastrobe 210 is early or late (high or low). In some embodiments, thisoccurs if the timing offset is within ±1 bit time (±0.5 cycles of theclock signal 114 in FIG. 1A) of the target timing offset. As discussedbelow with reference to FIGS. 4-5, if the timing offset is larger thanthis range, in some embodiments additional circuits are used during thefirst mode of operation to achieve this level of tracking accuracy. Thecircuit 200 also uses data strobe burst error information, which isdefined as any time a memory read is completed and a data strobe edgecounter did not count two positive or rising edges in the data strobe210. A relative delay between the strobe enable signal 172 and the datastrobe 210 is determined based on the combination of the burst errorinformation and the data strobe sample information. In particular, thedelay of the strobe enable signal 172 is increased if the burst error orthe data strobe sample are high (i.e., the early condition), anddecreased if the burst error and the data strobe sample are low (i.e.,the late condition). A feedback loop described below is updated at theend of a sequential read burst, i.e., at the falling edge of the enablesignal 172. This ensures that adjustment of the delay occurs when readtraffic is not present.

In the circuit 200, the data strobe 210 is received on a node in aninterface and is coupled to on-die termination (ODT) biasing circuit 212and receive circuit 214. As described further below with reference toFIG. 2A, in some embodiments the on-die termination biasing circuit 212biases a voltage level in the interface during the first mode ofoperation based on a lock signal that is provided by lock detect circuit226. This biasing may shift the data strobe 210 waveform and removenoise-induced edges during initial acquisition, i.e., during the firstmode of operation. An output from the receive circuit 214 is the datastrobe internal signal 170 (dqs_int). This signal is coupled to latch216-2 and AND gate 218. It is also used to clock latch 216-1.

Delay adjust circuit 224 outputs the enable signal 172 when read enableis asserted or is logical high. As discussed below with reference toFIG. 2D, the delay adjust circuit 224 provides the appropriate delay ortiming offset to synchronize the data signal and the data strobe 210. Inan exemplary embodiment, the delay adjust circuit 224 provides delays upto 3.875 cycles of the clock signal 114 (FIG. 1A) in steps of ⅛ of acycle. The enable signal 172 is coupled to the AND gate 218 by the latch216-1 on the rising edges of the data strobe internal signal 170. Theenable signal 172 is also coupled to the latch 216-2, the lock detectcircuit 226, and burst error detect circuit 228.

The output from gate 218 is the clean version of the data strobe 174,dqs_clean. Delay element 220 provides a 90° phase shift, whichapproximately centers the transitions in the clean version of the datastrobe 174 in the middle of a bit cell or half cycle of the clock signal114 (FIG. 1A), i.e., in the middle of the data eye pattern. The outputfrom the delay element 220 is coupled to data capture logic 250 and toFIFO IPTR Counter 222. FIFO IPTR Counter 222 counts the edges in theclean version of the data strobe 174, and outputs a Write Select signal223 that is a repeating sequence of gray codes: 00, 01, 11, 10 (whichthen repeats), incrementing the gray code in response to each upwardtransition of the clean strobe signal dqs_clean 174. Write Select 223,output from the FIFO IPTR Counter 222, directs the writing of receiveddata (received in parallel with the strobe signal) into the slots of aFIFO buffer (not shown). Write Select 223 is also coupled to the bursterror detect circuit 228.

As described below with reference to FIG. 2C, the burst error detectcircuit 228 determines if a burst error has occurred in the data strobe210 based on the Write Select signal 223. In particular, if the datastrobe 210 is appropriately centered in the window corresponding to theenable signal 172, the clean strobe signal will have four edges,including two rising edges, during each data burst (FIG. 1B). If this isnot the case, a burst error is asserted on the output from the bursterror detect circuit 228 (burst error), which is coupled to OR gate 230and error handling logic. The error handling logic may be included inthe control logic 136 (FIG. 1A), and may be used to control theacquisition process. For example, in response to assertion of a bursterror, the memory controller 110 (FIG. 1A) may issue another orsupplemental read command.

The latch 216-2 samples the data strobe 210 at the falling edges of theenable signal 172 in order to determine if the data strobe 210 is earlyor late relative to the window. Referring to FIG. 1C, if the fallingedge of the enable signal dqs_en 172 coincides with the internal strobesignal dqs_int at a low state, this indicates that the entire strobesignal falls within the enable window. This is called “the latecondition” because the end of the enable window is later than the end ofthe internal strobe signal, as it should be during normal operation. Onthe other hand, if the falling edge of the enable signal dqs_en 172coincides with the internal strobe signal dqs_int at a high state, thisis called “the early condition” because the enable window ends earlierthan the end of the strobe signal. The output from the latch 216-2(dqs_en_early) is received at an input of the OR gate 230.

The delay provided by the delay adjust circuit 224 is updated and/ormodified based on the output from the OR gate 230 (which, in turn,depends on the outputs from the burst error detect circuit 228 and thelatch 216-2). Thus, the delay adjust circuit 224 provides closed loopfeedback. In some embodiments, this feedback is based on a combinationof data strobe error and data strobe position information, and is usedto adjust the phase or timing of the enable signal 172 window relativeto the data strobe signal. As discussed previously, in these embodimentsthe delay of the enable signal 172 is increased if the burst error(burst_error) or the data strobe sample (dqs_en_early) are high (i.e.,the early condition), and decreased if the burst error (burst error) andthe data strobe sample (dqs_en_early) are both low (i.e., the latecondition).

The lock detect circuit 226 determines if synchronization has beenachieved (i.e., if the timing offset is less than one-half of a cycle ofthe clock signal 114 in FIG. 1A) based on the output from the OR gate230 and the enable signal 172. The lock detect circuit 226 outputs thelock signal to the on-die termination biasing circuit 212 and the errorhandling logic. In particular, if synchronization is achieved, in someembodiments the lock signal returns the on-die termination biasingcircuit 212 to normal operation.

FIG. 2B is a block diagram illustrating an embodiment of the lock detectcircuit 226. The output from the OR gate 230 (FIG. 2A) is inverted byinverter 234, and the resulting inverted signal is coupled to an inputof OR gate 232. Latch 216-3 couples this signal back to the OR gate 232based on the falling edges of the enable signal 172. If the output fromthe OR gate 232 (FIG. 2A) is low, i.e., the data strobe 210 is neitherearly nor late and a burst error is not asserted, then the output of theOR gate 232 will be high, corresponding to a lock or synchronizationcondition. Latch 216-3 is reset to an initial state when a Reset signalis asserted (e.g., by a processor or control logic in the memorycontroller).

FIG. 2C is a block diagram illustrating an embodiment of a burst errordetect circuit 228. The two bit Write Select 223 (FIG. 2A) is stored ina latch 236, which is clocked at the end of each enable window by adownward transition of the enable signal dqs_en 172. The remainder ofthe burst error detect circuit 228 can be divided into two branches: aleft branch and a right branch. The left branch determines if the finalvalue of Write Select 223 at the end of the enable window is a legalvalue (00 or 11). If the latched Write Select is a legal value,iptr-error is set to 0, and if the latched Write Select is an illegalvalue iptr-error is set to 1. This function is accomplished by an XORgate 238-2, which exclusive-OR's the [0] and [1] bits of the latchedWrite Select.

The right branch of the burst error detect circuit 228 determines ifWrite Select 223 advanced from one value to another during the lastenable window. In particular, the right branch generates a Burst signalthat is equal to 1 if Write Select 223 advanced from one value toanother during the last enable window (i.e., one or more data strobeswere received during the enable window). An OR gate 240 logically OR'sthe inverse of the Burst signal and the iptr-error signal to produce aburst_error output signal, which is equal to a logical “1” if a bursterror has occurred. In other words, if a burst error has occurred, theoutput from the burst error detect circuit 228 is asserted.

In one embodiment, the Burst signal is produced as follows. An XOR gate238-1 exclusively-OR's the [0] bit of the current Write Select 223 andthe [0] bit of the latched Write Select. The output of the XOR gate238-1 is equal to 1 if its inputs differ in value, which means that theoutput of the XOR gate 238-1 is equal to 1 if Write Select has advancedto a new value since the end of the last enable window. A One-Catchercircuit 239 detects if the output of the XOR gate 238-1 is equal to 1anytime during an enable window. It does this by outputting a 1 if a 1is detected on the output of the XOR gate 238-1. At the end of eachenable window, the output of the One-Catcher 239 is stored in latch 237and the One-Catcher 239 is reset. The One-Catcher 239 includes a latchthat captures the input of the One-Catcher 239 whenever the inputtransitions from a 0 to a 1. The latch of the One-Catcher 239 is clearedwhen the enable window is inactive (e.g., dqs_en is 0).

The output of latch 237 is the aforementioned Burst signal, which isequal to 1 if Write Select changed in value at least once during theprior enable window. An inverter 241 inverts the Burst signal to producea No Burst signal that is logically OR'ed with the iptr-error signal byOR gate 240 to produce the output of the burst error detect circuit 228,burst error.

FIG. 2D is a block diagram illustrating an embodiment of a delay adjustcircuit 224. If the output from the OR gate 230 (FIG. 2A) is high, theprior delay value plus a positive offset is applied by multiplexer (MUX)244 to latch 246 (i.e., the delay produced by delay element 242 isincreased). And if the output from the OR gate 230 is low, a negativeoffset is applied by multiplexer 244 to latch 246 (i.e., the prior delayproduced by delay element 242 is decreased by an offset). The latch 246updates or modifies the setting of delay element 242 (for example, astored register value) at the end of each enable window (i.e., at thefalling edges of the enable signal 172). In this way, the feedbackresults are applied to the next data strobe. The delay element 242outputs the enable signal 172 when read enable is high or is asserted.

In some embodiments, the circuit 200, the lock detect circuit 226, theburst error detect circuit 228, and/or the delay adjust circuit 224 mayinclude fewer or additional components, logical positions of one or morecomponents may be changed, and two or more of the components may becombined and/or shared. For example, in some embodiments the circuit 200may not include the burst error detect circuit 228, the OR gate 230, andthe lock detect circuit 226. Furthermore, in another embodiment thesampling time of the data strobe 210 is adjustable. And in anotherembodiment, the on-die termination biasing circuit 212 duringacquisition (i.e., during the first mode of operation) is optimizedusing an additional control loop.

We now discuss several embodiments of locking techniques that may beused in the first and/or the second mode of operation. FIG. 3A shows atiming diagram 300 related to the operation of the circuit shown in FIG.2A. During the first mode of operation, a voltage reference V_(ref),illustrated by the dashed line, that defines a voltage level in theinterface is biased, for example, using the on-die termination biasingcircuit 212 (FIG. 2A). This shifts the input data strobe 310 waveform(V_(tf)) down and removes or reduces noise induced edges from datastrobe internal 312. However, the data strobe internal 312 and cleanversion of the data strobe 316 are now asymmetric and have distortededges.

A minimum value of the timing offset or delay of enable signal 314 inthe delay adjust circuit 224 (FIG. 2A) is initially used. Two or morenon-continuous read commands are issued by the memory controller 110(FIG. 1A). Note that during this acquisition phase the actual read datais not important. In some embodiments, the delay is progressivelyincreased, thereby shifting the enable signal 314 to the right until thedelay is decreased for the first time (i.e., the enable signal 314 hasovershot the target timing offset or delay). In other embodiments, thedelay is progressively increased, thereby shifting the enable signal 314to the right until four edges in the data strobe internal 312 arecounted. Note that timing ranges 318 and 320 of delay adjustment in thetiming diagram 300 reflect the initial use of a minimum delay settingfor the timing offset. Once the delay has been reduced for the firsttime, or the four edges of the data strobe are counted, the on-dietermination biasing circuit 212 (FIG. 2A) is returned to normaloperation, and tracking in second mode of operation is used.

FIG. 3B is another exemplary timing diagram 350 for the circuit shown inFIG. 2A. The voltage reference V_(ref) that defines the voltage level inthe interface has been returned to its normal value, i.e., its value inthe second mode of operation. Data strobe 360 waveform is shifted backup, resulting in noise edges on enable signal 362. However, the enablesignal 362 and clean version of the data strobe 366 are now symmetricand no longer have distorted edges.

The tracking loop in timing diagram 350 is unchanged relative to timingdiagram 300 (FIG. 3A). However, the read density must be adequate toprevent drift greater than ±1 bit time. Thus, the delay of enable signal364 is adjusted (dithered) within timing region 368 about the targetdelay, which is indicated by the vertical arrow. If the delay exceedsthis bound, the first mode of operation may be used again. In someembodiments, the output from the FIFO IPTR Counter 222 (FIG. 2A) ismonitored and used to initiate error routine handling (e.g.,transitioning to the first mode of operation) if the end of the enablewindow falls outside the timing region 368.

In other embodiments, a voltage level in the interface is not biasedduring the first mode of operation. Instead, a known or predefined datapattern (such as a pseudo-random sequence) may be used to lock the datastrobe and the enable signal. For example, the delay or timing offsetmay be swept over a range of values (from early to late or from late toearly) as the predefined data pattern is transmitted by the memorydevice 124 (FIG. 1A). The comparison circuit 134 (FIG. 1A) may comparethe received data with either a stored version of the predefined patternor a version of the predefined pattern that is generated in the memorycontroller 110 (FIG. 1A). When errors are detected, the pass-failboundaries and corresponding delays are determined. This is illustratedby the eye pattern 400 in FIG. 4. At extrema (maximum or minimum) of thetiming offset or delay, errors occur. These are indicated by pass-failboundaries 410.

One or more of the pass-fail boundaries 410 may be used to determine adelay that corresponds to the desired lock condition. For example, theaverage of the delays at the pass-fail boundaries 410 may be used. Thisdelay is approximately in the center of the eye pattern 400.Alternatively, once an edge in the data strobe is determined based onone of the pass-fail boundaries 410, a given offset may be added orsubtracted to obtain the delay that is approximately in the center ofthe eye pattern 400. Once the appropriate delay is determined and lockhas been acquired, the memory controller 110 (FIG. 1A) may transition tothe second mode of operation and the tracking technique illustrated inthe timing diagram 350 (FIG. 3B) may be used. Note that in someembodiments that utilize the approach illustrated in FIG. 4, the circuit200 (FIG. 2A) may not include the burst error detect circuit 228 (FIG.2A), the OR gate 230 (FIG. 2A), and/or the lock detect circuit 226 (FIG.2A).

In some embodiments, the timing offset is determined by comparing atleast a portion of the received read data with a predefined pattern, forexample, one that corresponds to a preamble of the data strobe. In theseembodiments, therefore, the delay during the acquisition or first modeof operation is initially biased late (as opposed to early). FIGS. 5Aand 5B are block diagrams illustrating embodiments of circuits 500 and550 that utilize such an approach. The circuits 500 of FIG. 5A are inthe PClk domain, while the circuits 550 of FIG. 5B are in the PClkFdomain. In some embodiments, a single instance of circuit 500 andcircuit 550 is shared by all the bits whose receipt is controlled by asingle data strobe. All the signals shown in circuits 500 and 550 aresingle bit wide signals, unless otherwise indicated in the figures orthe explanation below.

In the circuit 500, a counter 534 is initially set to maximum value,i.e., which corresponds to a maximum delay between the read command RD116 and the beginning of the enable signal window EN (see FIGS. 6A-6D).The counter outputs a coarse delay (DlyC) 512 and a fine delay (DlyF)518, and together these two values represent a variable delay. A maximumdifference between the initial value of the variable delay (set at thebeginning of the first mode of operation) and a final value of thevariable delay (at the end of the first mode of operation, which occurswhen a match is detected between the data strobe and a predefined datastrobe pattern) is typically more than two clock cycles of the PClksignal. This enables the receiving device to be able to handle verylarge variations in the read cycle round trip timing.

Latches 510 are used to provide delayed versions of the read command 116to multiplexer 514. A respective delayed version of the read command,referred to as RDc 524, is output by the multiplexer 514 based on thecoarse delay DlyC[2:0] 512. Latches 526 provide delayed and un-delayedversions of RDc 524 to skip multiplexer 528. The output from the skipmultiplexer 528, which is referred to as RDf 530, is selected based onthe fine delay DlyF 518 (e.g., the most significant bit DlyF[2] of thefine delay).

Offset clock generator 516 provides four clock signals (PClk00, PClk01,PClk10, PClk11) having different phase offsets relative to the PClkclock signal 114. PClk00, PClk01, PClk10, and PClk11 are delayed withrespect to the PClk signal by 0, 0.25, 0.5, and 0.75 clock cycles,respectively. The four clock signals are coupled to multiplexer 520. Themultiplexer 520 outputs a clock with a respective phase, which isreferred to as PClkF 522, based on the value of the fine delay DlyF 518(e.g., based on least significant bits DlyF[1:0] of the fine delay).

RDc 524 (i.e., the appropriately delayed version of the asserted readcommand) is also used to enable an update of the counter 534 after beingdelayed by delay element 532. This delay ensures that updates occur whenread traffic is not present. In an exemplary embodiment, the minimumread burst size is four clock cycles (of PClk 114) in length and thedelay element 532 corresponds to ten cycles of the clock signal 114.Note that the components illustrated in the circuit 500 are clockedusing the clock signal 114, i.e., they are in the PClk domain. Thecomponents in the circuit 550, however, are clocked using PClkF 522,i.e., they are in the PClkF domain. As discussed further below, thephase of the PClkF domain tracks the phase of the data strobe.

Referring to FIG. 5B, the circuit 550 has two modes of operation. In thefirst mode of operation, INIT 566 is asserted and the delay (DlyC andDlyF) is initially set to its maximum possible value. Latches 560 inconjunction with OR gates 562 and 568 and AND gate 564 provide enablesignal (EN) 570, which defines the enable window. When any of the inputsto OR gate 568 is a logical 1, the OR gate 568 sets the enable signal570 to a logical 1. The enable signal 570 window in the first mode ofoperation is 10.5 clock cycles wide. This is accomplished by issuing twoback-to-back transactions, each with four cycles of data, with 1.5cycles of preamble and 1.0 cycles of post-amble.

Latches 572 in conjunction with OR gate 578, AND gates 576, 580 and 582,flip-fop (FF) 584, and XOR gate 586 compare data strobe 574 with anexpected or predefined pattern (e.g., 001010, where the left most bit ofthis pattern is the first or oldest bit of the pattern). In the firstmode of operation, the comparison is based on six samples. The XOR gate586 outputs an increment/decrement signal 536 to the counter 534 (FIG.5A). In the embodiment shown in FIGS. 5A and 5B, signal 536 indicates“increment” or “up” when the signal 536 is equal to 0 and indicates“decrement” or “down” when the signal 536 is equal to “1”. Since thedelay is initially set to the maximum value, in the first mode ofoperation the circuit 550 will progressively decrease the delay until alock condition is acquired. More specifically, in the first mode ofoperation, INIT=1, which causes the XOR gate 586 to output anincrement/decrement signal 536 that is equal to the Match signal (whichis a logical 1 when the strobe signal matches the predefined pattern).Thus, in the first mode of operation, the increment/decrement signal 536is set to decrement (i.e., to a logical 1) whenever the strobe signalDQS 574 does not match the predefined pattern (i.e., when Match=0).

Once the lock is acquired, the circuit 550 switches to a second mode ofoperation in which INIT 566 is set low (INIT=0). In some embodiments,the first several read cycles in the second mode of operation are usedto adjust the strobe enable window to the dither point, after whichnormal data operations can begin. In other embodiments, normal datatransmission can begin as soon as the circuit 550 switches to the secondmode of operation. In the following discussion, FIGS. 6C and 6D aretiming diagrams representing the first few read cycles after enteringthe second mode of operation, while FIG. 6E represents normal operationafter the dither point has been reached. The transition from the “adjusttiming” sub-mode to the “normal” sub-mode of operation (both of whichare sub-modes of the second mode of operation and have INIT=0) occurswhen the increment/decrement signal 536 equals “increment” for the firsttime while in the second mode of operation.

In the second mode of operation, the width of the enable signal 570 is4.5 clock cycles, and the comparison of the data strobe 574 with thepredefined pattern utilizes four samples (e.g., 1010) instead of six. Inthis mode of operation, the feedback loop adjusts the delay (dithers)about the target delay value. When there is a pattern match (Match=1),the delay is decreased, and when there is no pattern match (Match=0) thedelay is increased. As in the other embodiments described previously, ifexcessive drift occurs (i.e., the lock condition is not maintained towithin ±1 bit time), the memory controller 110 (FIG. 1A) may switch thecircuit 550 to the first mode of operation to reacquire the proper delayor phase setting for the strobe enable signal.

In some embodiments, the circuits 500 (FIG. 5A) and/or 550 include feweror additional components, logical positions of one or more componentsmay be changed, and two or more of the components may be combined and/orshared.

The operation of the circuits 500 (FIG. 5A) and 550 is illustrated inFIGS. 6A-6E. FIGS. 6A and 6B are timing diagrams for a first mode ofoperation (INIT=1), and FIGS. 6C, 6D and 6E are timing diagrams for thesecond mode of operation (INIT=0). Note that in FIGS. 6A-6D, two or moreread burst operations have been initiated. The second read command (andany subsequent read commands) results in a normal read operation in thememory device 124 (FIG. 1A), including a data strobe and read data beingreturned. However, in FIGS. 6A and 6B (which correspond to the firstmode of operation, with INIT=1) only the first read command initiates ahigh pulse or value for one clock cycle of RDc 524 and extends thewindow corresponding to the enable signal 570. The second read commandis shown in the FIGS. 6A-6D by a RD=0 signal that occurs four clockcycles after the first read command. Because the second read commanddoes not cause an RD=1 input to circuitry 500, the second read commanddoes not extend the strobe enable window. The additional read commandsensure that there are valid data strobe 574 signal levels after thefirst read operation. The memory controller 110 (FIG. 1A), therefore, iscapable of supplying and distinguishing between these two differenttypes of read commands during the first mode of operation.

FIG. 6A is a block diagram illustrating an embodiment of a timingdiagram 600. The first read command is delayed by coarse delay 610 andasserted for one clock cycle as RDc 524. The appropriate phase of theclock signals output by the offset clock generator 516 (FIG. 5A) isselected. In this case, it is PClk11. The multiplexer 528 (FIG. 5A)applies skip delay 612 in accordance with the DlyF[1] (the mostsignificant bit of DlyF 614) to produce RDf 530. RDf 530 is then delayedin accordance with DlyF 614 by synchronizing it with PClkF in latches560 (FIG. 5B) to produce the enable signal 570 window EN.

Six samples of the data strobe 574 are compared to an expected pattern(represented by a set of six dots synchronized to the upward transitionsof the PClkF clock signal) during the enable signal 570 window EN, whichis 6.5 clock cycles wide. Since the received pattern and the expectedpattern do not match (Match=0), and INIT=1 (first mode of operation),increment/decrement signal 536 is set to decrement (1). In the firstmode of operation (INIT=1), when Match=0 (indicating the enable windowis not yet properly positioned), the increment/decrement signal 536 isset equal to decrement (1), and when Match=1 (indicating the enablewindow is properly positioned), the increment/decrement signal 536 isset equal to increment (0).

Eventually, the delay is reduced sufficiently that the received patternand the expected pattern match. This is illustrated in timing diagram620 of FIG. 6B. Coarse delay 621 is now four clock cycles, the skipdelay 622 is a half clock cycle, and the fine delay 624 is 0.75 clockcycles. As a result, the beginning of the enable signal window EN isdelayed by 1.25 clock cycles (0.5+0.75 clock cycles) from the beginningof the RDc signal 524. The appropriate phase of the clock signal outputby the offset clock generator 516 (FIG. 5A) is PClk01 (0.25 clock cyclesdelayed from PClk). As shown, the six samples of the data strobe 574match the expected pattern (Match=1), including the preamble of the datastrobe 574. The circuit 550 (FIG. 5B) is, therefore, switched to thesecond mode of operation. In addition, increment/decrement is set to 0(increment), causing the delay to be increased by a quarter clock cycle.

The second mode of operation (INIT=0) is illustrated by the timingdiagrams 640, 650 and 680 in FIGS. 6C, 6D and 6E. Note that the enablesignal 570 window EN is now 4.5 clock cycles wide and four samples areused when comparing the data strobe 574 and the expected pattern. In thesecond mode of operation (INIT=0), when there is a pattern match (asindicated by Match=1), increment/decrement 536 is set to decrement (1)(as shown in FIGS. 6C and 6E), and when there is no pattern match,increment/decrement 536 is set to increment (0) (as shown in FIG. 6D).This results in dithering about the desired delay target. This isillustrated in the timing diagrams 650 and 680 of FIGS. 6D and 6E. FIG.6C represents a timing diagram 640 of an exemplary read cycle that isused for adjusting the enable window 570, in the second mode ofoperation, prior to reaching the dither point. In the particular, atthis point of the timing adjustment process, the receiving device hasincremented DlyC/DlyF once during the first read cycle of the secondmode of operation, and then decremented DlyC/DlyF once at the next readcycle, thereby returning DlyC/DlyF to the same value as it had at thetermination of the first mode of operation. In FIG. 6C, the coarse delay630 is four clock cycles, skip delay 622 is a half cycle, and the finedelay 624 is 0.75 clock cycles, for a total delay of 5.25 clock cycles.Furthermore, the increment/decrement signal 536 is equal to decrement(1) because the DQS signal that falls within the enable window matchesthe predefined pattern (Match=1). During subsequent read cycles,DlyC/DlyF will continue to be decremented until the DQS signal thatfalls within the enable window no longer matches the predefined pattern(Match=0), as represented by timing diagram 680 in FIG. 6D. In FIG. 6D,coarse delay DlyC 660 is three clock cycles, skip delay 662 is a fullcycle, and the fine delay DlyF 664 is 0.50 clock cycles. In FIG. 6D thedelay from the read command RD to the enable signal window EN, 4.5 clockcycles, is 0.75 clock cycles less than the delay in FIG. 6C. BecauseMatch=0, increment/decrement signal 536 changes from decrement (1) toincrement (0), which means the circuit has reached the dither point.After reaching the dither point, the delay will be alternately decreasedand increased, as represented by FIG. 6E, so long as the relative timingof the data strobe and the read command signal remain unchanged.

We now discuss embodiments of processes for reducing and/or eliminatingtiming drift. FIG. 7 is a flow diagram illustrating an embodiment 700 ofa method of timing drift cancellation. This method may be implementedusing any of the circuits and systems described above, as well as othercircuits or systems that utilize the techniques described herein. A datastrobe signal and corresponding read data are received (710). The datastrobe signal and the read data correspond to a read command, and theread data is received in accordance with the data strobe signal and anenable signal. A timing offset between the enable signal and the datastrobe signal is dynamically adjusted (712). A supplemental read commandis issued if a time interval since a last read command issued by amemory controller exceeds a predetermined value (714). In someembodiments, there may be fewer or additional operations, an order ofthe operations may be rearranged, and/or two or more operations may becombined.

Devices and circuits described herein can be implemented using computeraided design tools available in the art, and embodied by computerreadable files containing software descriptions of such circuits, atbehavioral, register transfer, logic component, transistor and layoutgeometry level descriptions stored on storage media or communicated bycarrier waves. Data formats in which such descriptions can beimplemented include, but are not limited to, formats supportingbehavioral languages like C, formats supporting register transfer levelRTL languages like Verilog and VHDL, and formats supporting geometrydescription languages like GDSII, GDSIII, GDSIV, CIF, MEBES and othersuitable formats and languages. Data transfers of such files on machinereadable media including carrier waves can be done electronically overthe diverse media on the Internet or through email, for example.Physical files can be implemented on machine readable media such as 4 mmmagnetic tape, 8 mm magnetic tape, 3½ inch floppy media, CDs, DVDs andso on.

FIG. 8 is a block diagram illustrating an embodiment of a system 800 forstoring computer readable files containing software descriptions of oneor more of the circuits described above. The system 800 may include atleast one data processor or central processing unit (CPU) 810, memory814 and one or more signal lines or communication busses 812 forcoupling these components to one another. Memory 814 may includehigh-speed random access memory and/or non-volatile memory, such as oneor more magnetic disk storage devices. Memory 814 may store a circuitcompiler 816 and circuit descriptions 818. Circuit descriptions 818 mayinclude circuit descriptions for the circuits, or a subset of thecircuits discussed above with respect to FIGS. 1-6. In particular,circuit descriptions 818 may include circuit descriptions of one or morecontrollers 820, one or more memory devices 822, control logic 824, oneor more transmit circuits 826, one or more receive circuits 828, one ormore interfaces 830, memory or memory cores 832, one or more latches834, one or more terminators 836, one or more delay elements 838, one ormore logic elements 840, one or more counters 842, one or moremultiplexers 844, and/or one or more clock generators 846.

The foregoing descriptions of specific embodiments of the presentinvention are presented for purposes of illustration and description.They are not intended to be exhaustive or to limit the invention to theprecise forms disclosed. Rather, it should be appreciated that manymodifications and variations are possible in view of the aboveteachings. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated.

What is claimed is:
 1. A memory controller, comprising: an interface toreceive a data strobe signal and corresponding read data, wherein thedata strobe signal and the read data correspond to a read command issuedby the memory controller, and wherein the read data is received inaccordance with the data strobe signal; a circuit to dynamically adjusta timing offset between the read data and an internal signalcorresponding to the data strobe signal; a lock detect circuit to detectwhen the timing offset is less than a predefined threshold and to outputa lock signal when the timing offset is less than the predefinedthreshold; and an on-die termination circuit configured to receive thedata strobe signal and lock signal and to enable a predefined normalmode of operation of the on-die termination circuit for terminating thedata strobe signal, in response to the lock signal, when the lock detectcircuit detects that the timing offset is less than the predefinedthreshold.
 2. The memory controller of claim 1, wherein thepredetermined threshold is one-half of a clock cycle of a clock used totransmit or receive data to or from one or more memory devices.
 3. Thememory controller of claim 1, wherein the circuit to dynamically adjustthe timing offset is configured to generate at least one early/latesignal indicating whether the received data strobe signal is early orlate relative to a timing window for a read enable signal of the memorycontroller, and to increase or decrease the timing offset in accordancewith a current value of the early/late signal.
 4. The memory controllerof claim 1, further comprising a data strobe burst error detectioncircuit for generating a burst error signal when timing of the datastrobe relative to a data strobe enable signal fails to satisfypredefined criteria, wherein the burst error signal is coupled to inputsof the lock detect circuit and the circuit to dynamically adjust thetiming offset.
 5. The memory controller of claim 1, wherein the internalsignal corresponding to the data strobe signal comprises a secondinternal signal corresponding to the data strobe signal; and the on-dietermination circuit is configured to operate in a first mode ofoperation, to bias a voltage level so as to shift a first internalsignal corresponding to the data strobe signal, distinct from the secondinternal signal corresponding to the data strobe signal, when the lockdetect circuit detects that the timing offset is not less than thepredefined threshold.
 6. The memory controller of claim 5, furthercomprising a receive circuit having an input coupled to an output of theon-die termination circuit and an output that outputs the first internalsignal corresponding to the data strobe signal; and a gate circuithaving an input coupled to receive the first internal signalcorresponding to the data strobe signal and an output that outputs thesecond internal signal corresponding to the data strobe signal, whereinthe gate circuit filters out spurious transitions in the first internalsignal corresponding to the data strobe signal.
 7. The memory controllerof claim 1, further comprising a gate circuit to filter out spurioustransitions in a first internal signal corresponding to the data strobesignal so as to generate said internal signal corresponding to the datastrobe signal.
 8. The memory controller of claim 1, further comprisingcircuitry that receives the internal signal corresponding to the datastrobe signal and, in response, outputs a write select signal, whichdirects writing of received data received by the memory controller. 9.The memory controller of claim 1, further comprising counter circuitrythat counts edges in the internal signal corresponding to the datastrobe signal and outputs a write select signal, which directs writingof received data received by the memory controller.
 10. The memorycontroller of claim 9, further comprising a data strobe burst errordetection circuit, coupled to an output of the counter circuitry, thedata strobe burst error detection circuit configured to generate a bursterror signal when the count of edges by the counter circuitry for apredefined data burst does not match a predefined value.
 11. A method ofoperating a memory controller, comprising: receiving, at an interface ofthe memory controller, a data strobe signal and corresponding read data,wherein the data strobe signal and the read data correspond to a readcommand issued by the memory controller, and wherein the read data isreceived in accordance with the data strobe signal; dynamicallyadjusting a timing offset between the read data and an internal signalcorresponding to the data strobe signal; detecting when the timingoffset is less than a predefined threshold; in response to thedetecting, outputting a lock signal when the timing offset is less thanthe predefined threshold; and in response to the lock signal, which isoutput when the timing offset is less than the predefined threshold,enabling a predefined normal mode of operation of an on-die terminationcircuit, for terminating the data strobe signal received at theinterface.
 12. The method of claim 11, wherein the predeterminedthreshold is one-half of a clock cycle of a clock used to transmit orreceive data to or from one or more memory devices.
 13. The method ofclaim 11, wherein dynamically adjusting the timing offset includesgenerating at least one early/late signal indicating whether thereceived data strobe signal is early or late relative to a timing windowfor a read enable signal of the memory controller, and increasing ordecreasing the timing offset in accordance with a current value of theearly/late signal.
 14. The method of claim 11, further comprisinggenerating a burst error signal when timing of the data strobe relativeto a data strobe enable signal fails to satisfy predefined criteria,wherein the burst error signal is coupled to circuitry in the memorycontroller for detecting when the timing offset is less than thepredefined threshold and to circuitry in the memory controller fordynamically adjusting the timing offset.
 15. The method of claim 11,wherein the internal signal corresponding to the data strobe signalcomprises a second internal signal corresponding to the data strobesignal; and the method further includes operating the on-die terminationcircuit in a first mode of operation, to bias a voltage level so as toshift a first internal signal corresponding to the data strobe signal,distinct from the second internal signal corresponding to the datastrobe signal, when the timing offset is not less than the predefinedthreshold.
 16. The method of claim 15, further comprising generating thefirst internal signal corresponding to the data strobe signal from anoutput of the on-die termination circuit; and filtering out spurioustransitions in the first internal signal corresponding to the datastrobe signal to produce the second internal signal corresponding to thedata strobe signal.
 17. The method of claim 11, further comprisingfiltering out spurious transitions in a first internal signalcorresponding to the data strobe signal so as to generate said internalsignal corresponding to the data strobe signal.
 18. The method of claim11, further comprising, in response to the internal signal correspondingto the data strobe signal, outputting a write select signal, whichdirects writing of received data received by the memory controller. 19.The method of claim 11, further comprising counting, with countercircuitry in the memory controller, edges in the internal signalcorresponding to the data strobe signal and outputting, in accordancewith the counting, a write select signal, which directs writing ofreceived data received by the memory controller.
 20. The method of claim19, further comprising generating a burst error signal when the count ofedges by the counter circuitry for a predefined data burst does notmatch a predefined value.